The present invention relates to room temperature vulcanizable silicone rubber compositions and more particularly the present invention relates to one-component room temperature vulcanizable silicone rubber composition which are shelf-stable in the uncured state.
One-component room temperature vulcanizable silicone rubber compositions are well-known. Generally, such compositions comprise basic ingredients, a silanol terminated diorganopolysiloxane polymer, a filler which may either be a silica filler or one of the extending fillers, a crosslinking agent which may be an acyloxy functional silane or an alkoxy functional silane and a metal salt catalyst. When the cross-linking agent is an acyloxy functional silane, the catalyst is the metal salt of a carboxylic acid, where the metal varies from lead to manganese in the Periodic Table. When the cross-linking agent is an alkoxy functional silane, then the catalyst is a titanium chelate catalyst. In the present application we are concerned with an acyloxy functional silane cross-linking agent. Most commonly, such a cross-linking agent is a methyl triacetoxy silane and the metal salt of a carboxylic acid is a tin salt.
With such a composition the ingredients are most commonly packaged in the anhydrous state or substantially anhydrous state. When it is desired to cure the composition, the seal on the container is broken and the composition is applied to whatever form it is desired and when it is exposed to atmospheric moisture it will hydrolyze and cross-link to form a silicone elastomer with total cure taking place in about 24 hrs. Examples of Prior Art Patents and Patents relating to such one-component room temperature vulcanizable silicone rubber composition are as follows:
Selin et al U.S. Pat. No. 3,730,932; PA1 Selin et al U.S. Pat. No. 3,941,74l; PA1 Selin et al U.S. Pat. No. 3,896,123; PA1 Selin et al U.S. Pat. No. 3,813,365; PA1 Selin et al U.S. Pat. No. 3,780,080; PA1 Selin et al U.S. Pat. No. 3,793,361; PA1 Selin et al U.S. Pat. No. 3,778,459; PA1 Selin et al U.S. Pat. No. 3,773,817; PA1 Selin et al U.S. Pat. No. 3,808,248; PA1 Selin et al U.S. Pat. No. 3,806,532; PA1 Dupree U.S. Pat. No. 3,274,145; PA1 A. H. Smith Ser. No. 16,255; PA1 M. D. Beers Ser. No. 919,544; PA1 Berger and Selin U.S. Pat. No. 3,759,968; PA1 Nitzsche and Wick U.S. Pat. No. 3,065,194; PA1 Bruner U.S. Pat. No. 3,035,016; PA1 Ceyzeriat U.S. Pat. No. 3,133,891; PA1 Brown et al U.S. Pat. No. 3,161,614; PA1 Cooper U.S. Pat. No. 3,383,355; PA1 Matherly U.S. Pat. No. 3,499,859; PA1 Cooper et al U.S. Pat. No. 3,542,901; PA1 Brown et al U.S. Pat. No. 3,122,522; PA1 Brown et al U.S. Pat. No. 3,170,894; PA1 Weyenberg U.S. Pat. No. 3,175,993; PA1 Smith and Hamilton U.S. Pat. No. 3,689,454; PA1 Smith and Hamilton U.S. Pat. No. 3,779,986; PA1 Weyenberg U.S. Pat. No. 3,294,739; PA1 Weyenberg U.S. Pat. No. 3,334,067; PA1 Clark et al U.S. Pat. No. 3,719,635; PA1 M. D. Beers U.S. Pat. No. 3,382,205; PA1 Berger U.S. Pat. No. 3,517,001; PA1 Smith and Beers U.S. Pat. No. 3,708,467; PA1 Lucas U.S. Pat. No. 2,938,009; PA1 Lichtenwalner U.S. Pat. No. 3,004,859; PA1 Smith U.S. Pat. No. 3,635,743.
The above patents disclose a number of additive and various other types of formulations which these one-component room temperature vulcanizable silicone rubber compositions can have.
Recently, however, there was developed a new type of one-component room temperature vulcanizable silicone rubber composition as disclosed in Beers Patent Application Ser. No. 919,544, filed June 27, 1978, and entitled "Curable Compositions and Processes and now abandoned." A composition disclosed in this patent application comprises as its basic ingredients a silanol chainstopped polydiorganosiloxane, a fluid polysiloxane having a high degree tri- or tetrafunctionality, a silica filler, a cross-linking agent silane which is acyloxy functional and a highly reactive tin containing silanol condensation catalyst. One of the novel aspects of this patent application was the disclosure for a cross-linking agent such as methyl tris(2-ethylhexanoxy) silane or methyl tris(benzoxy) silane. There was found that a one-component or one-part room temperature vulcanizable silicone rubber composition which had such a cross-linking agent in combination with a fluid polysiloxane having a high degree of tri- or tetrafunctionality; that such a composition produced a silicone elastomer that had high heat resistance, low corrosiveness and a slight odor. It should be noted that the compositions in which the cross-linking agent is methyl triacetoxy silane are objectionable to some people because of the strong odor of acetic acid that is given off by such compositions when they are curing to form a silicone elastomer.
Another application on a system that was an improvement in some ways of the Beers Application, Ser. No. 919,544, now abandoned, is that to be found in the Smith and Beers Patent Application 60 SI-223, Ser. No. 16,255 filed on Feb. 28, 1979 now U.S. Pat. No. 4,247,445. That Patent Application, Ser. No. 16,255 now U.S. Pat. No. 4,247,445, concerns the production of a paintable one-component room temperature vulcanizable silicone rubber composition which is paintable in the cured elastomeric state. One of the means for accomplishing this as disclosed in the Patent Application of Smith et al, Ser. No. 16,225 now U.S. Pat. No. 4,247,445, is the use of large amounts of calcium carbonate filler in the composition among the other improvements. However, in another vein, ther composition disclosed in Beers Ser. No. 919,544, now abandoned, did not have sufficient adhesion to various substrates such as plastic substrates and metal substrates.
It was highly desirable to have the composition of the foregoing Beers Ser. No. 919,544, now abandoned, be self-bonding, that is, that the composition bond to another substrate without the use of a primer by the incorporation of an additive into the composition when it was manufactured. It should be noted that Ser. No. 919,544, now abandoned, of the Beers Patent Application attempts to make the composition self-bonding and the composition is self-bonding to a certain extent by the use of silyl isocyanurates as adhesion promoter additives. An attempt was successfully made to improve the self-bonding characteristics of the Beers Ser. No. 919,544, (now abandoned) composition as disclosed in the Patent Application of Smith et al, Ser. No. 16,254 filed Feb. 28, 1979 now U.S. Pat. No. 4,273,698. This disclosure, that is, Ser. No. 16,254 now U.S. Pat. No. 4,273,698, discloses the use of silyl maleates, silyl fumarates and silyl succinates as adhesion promoters for the one-component room temperature vulcanizable silicone rubber composition which has an acyloxy functional silane cross-linking agent.
The use of such silyl maleates and silyl fumarates as adhesion promoters results in a one-component room temperature vulcanizable silicone rubber composition with improved adhesion properties to plastics and metals and self-bonding properties to plastics and metals over the prior composition of the Beers, Ser. No. 919,544 Patent Application now abandoned. However, there was one great disadvantage in the use of self-bonding adhesion promoters in such compositions. The disadvantage was that the shelf-like of the uncured composition was seriously degraded. Thus, by accelerated shelf-aging tests there were made on such compositions having the silyl maleates and even the silyl isocyanurate adhesion promoters in the uncured composition wherein the accelerated shelf-aging test the composition was stored at 50.degree. C. for as little as 15 days and for as long as 30 days at 50.degree. C.; it was found that under such conditions the strength of the composition would be lowered from 225 to 51 psi and would go to 0 psi after 30 days of accelerated shelf-aging at 50.degree. C. There was a comparable falling off the Shore A Hardness and also the Elongation in such accelerated shelf-aging test.
Accordingly, in such accelerated shelf-aging data that was obtained, it appears that adhesion promoters and specifically the silyl maleate, silyl fumarate, silyl succinate and silyl isocyanurate adhesion promoters would be difficult to use with the Beers, Ser. No. 919,544, (now abandoned) composition. It was only recently, unexpectedly discovered that by pre-reacting the cross-linking agent with an adhesion promoter, that this degradation in the physical properties of the cured composition could be avoided and that the composition would have good shelf-stability.
It is one object of the present invention to provide for a one-component room temperature vulcanizable silicone rubber composition which has good shelf-stability. It is an additional object of the present invention to provide for a one-component room temperature vulcanizable silicone rubber composition which has as adhesion promoter methyl tris(2-ethylhexanoxy) silane as a cross-linking agent.
It is yet an additional object of the present invention to provide for a one-component room temperature vulcanizable silicone rubber composition in which the adhesion promoter is pre-reacted with the cross-linking agent wherein pre-reacted reaction product is incorporated into the composition in order to make the composition shelf-stable.
These and other objects of the present invention are accomplished by means of the disclosure set forth herein below.